This invention is related to the sensing of sample temperatures at a microscopic level.
The measurement of temperature is fundamental to diverse fields of science and technology. Temperature affects the properties of matter and physical phenomena, governs the kinetics of chemical reactions, triggers biomolecular mechanisms and controls engineering processes. Some of today's most prominent technical challenges such as self-heating in electronics, sustainable energy conversion and information processing rely on in-depth understanding of the transfer of thermal energy and a quantification of temperature distributions in active devices.
Attempts to quantify thermal properties and processes at a microscopic level have used a scanning probe microscope in which a temperature sensor is provided at the tip of the probe. This technology is referred to as scanning thermal microscopy (SThM). In SThM systems, the probe tip is brought into contact or close proximity with a sample surface to establish a thermal coupling due to heat flux between the sample and the temperature sensor. To perform a scan, the probe is moved through multiple positions relative to the sample, e.g. by moving a platform supporting the sample. Circuitry associated with the sensor provides a signal indicating the response of the sensor to local temperature at each position.
SThM has been applied to probe thermal properties such as thermal conductivity and thermal processes such as phase transitions or Joule-heating. Different kinds of scanning probes, both actively-heated and passive, have been employed using various sensing elements such as fluorescence, thermocouple or thermoresistive sensors. Examples of such SThM systems are described in U.S. Pat. No. 5,441,343, and “Scanning probe methods for thermal and thermoelectric property measurements.” Further examples, in which samples are scanned in both contact and non-contact modes, are described in International Patent Application Publication No's. WO 2011/002201 A2 and WO 2012/165791 A2; and “Quantitative Measurement with Scanning Thermal Microscope by Preventing the Distortion Due to the Heat Transfer through the Air.” Some systems use both AC (alternating current) and DC (direct current) excitation of a thermoresistive sensor on the probe in order to heat the sensor.
All such temperature sensing systems face one common challenge in the quantification of thermal properties. The acquired measurement signal depends on the heat flux Q between the sensor and the sample and this is generally a function of two unknown quantities, the thermal conductance G and the temperature difference ΔT and between the sensor and sample. The problem is exacerbated when using very small probes designed to sense sample temperatures at high spatial resolution, as in SThM systems. This is because, on scaling down the size, the thermal conductance G scales down and becomes increasingly difficult to quantify.
In most prior attempts to quantify temperature fields using scanning probes, the thermal conductance is assumed to be constant, or approximated from literature or calibrated for a given probe. Consequently, the acquired temperature signal is disturbed by local variations of the thermal coupling between the sensor and the sample. This disturbance becomes apparent when the thermal coupling is changing, e.g. due to topography related changes in the size of the coupling area or material related changes in thermal conductance.
An attempt to quantify temperature using a scanning probe by nullifying the heat flux between the sensor and sample is described in “Ultra-high vacuum scanning thermal microscopy for nanometer resolution quantitative thermometry.” The heat flux between the sensor and sample is measured at a plurality of different temperatures of an actively heated sensor, and the sample temperature is extracted from interpolation of the sensor temperature to zero heat flux. This technique is inherently slow and complicated, and is unsuitable for use in a scanning mode of a microscope within acceptable signal acquisition times.
Another attempt to quantify temperature fields is described in “Quantitative thermometry of nanoscale hot spots.” The heat flux between the sensor and sample is first acquired for a known temperature difference ΔT, followed by a consecutive measurement of the heat flux at an unknown temperature difference. Changes in the heat flux are related to changes in the sample temperature. By first performing a reference measurement, this method requires two consecutive measurements and high reproducibility of the thermal coupling between the two measurements. This both inhibits practicality of the method and has adverse implications for reliability.